1. Field of the Invention
This invention relates to an improved method and apparatus for generating sequence-of-events data in a distributed process control system, and more particularly, to generating unfiltered sequence-of-events data that records as an event very rapid changes in a process signal input.
2. Description of the Prior Art
A distributed process control system of the type generally contemplated by this invention is disclosed in U.S. Pat. No. 4,304,001 to Cope assigned to the assignee of this invention and incorporated herein by reference. The patented Cope system has a number of remotely located process control units, each coupled through an associated input/output device to a communications link, which interconnects all the remote units. Each process signal input lead, which provides an "on" or "off" status of a process element, is terminated at a printed circuit card and each printed circuit card has, for example, a capacity to receive 16 process signal inputs. Each remote unit typically has a number of these process input printed circuit cards.
As will be appreciated by those skilled in the art, in a process control system it is highly desirable to be able to construct a sequence-of-events record in order to determine, in a time ordered sequence, the state of some or all of the process elements within a window of time. This sequence-of-events record allows an analysis of the process operation; for example, an analysis to determine the cause of a process upset.
To generate a sequence-of-events record, each change of status of a process element is noted, tagged with a time-of-day indication, and the status and its time tag stored at least for a short period. In order to provide a highly accurate sequence-of-events record, it is desirable to time tag and store changes in a process element status that occur in close time sequence, i.e., within 0.1 millisecond. In a distributed process control system, it is difficult to maintain close clock synchronization throughout the distributed network, and to gather and store a large amount of sequence-of-event data without overburdening the communications network linking the remote units or the I/O channel linking the inputs from the process elements to the network.